Running at full tilt!

Great result! By updating 96 entries in the (memory-mapped) palette map rather than the palette index in 1024 (effectively port-mapped) sprite registers, the entire ISR now runs for signifcantly less than 1/4 of a frame and the landscape drawing is finished well before the next VBLANK interrupt.

Palette update finished before the visible area of the frame is rendered

This gives plenty of head-room for the code to run and any potential race conditions should now be alleviated. The game should run at 100% now with no glitches.

One thing I did (also) change is move the scrolling to the start of the ISR (before palette change) to eliminate the screen tearing I noticed on MAME. I'll do the same for the sprites.

And the background colour; I was thinking I'd need another layer of sprites but of course you can control the background colour directly on the Neo Geo (as I have been doing to profile the ISR) so that was a one-line implementation!

Blue background. All that is missing are the stars

Next up stars (which will need their own layer - just another 32 sprites and some tiles created for stars) and maybe use palette to blink them.

Finally, I need to tweak some Neo Geo-specific features, such as coining up and playing a game. At the moment I can only read the SELECT button running as an AES. Not sure how to handle coin-up on MVS just yet as it's handled by the BIOS depending on what mode the game is running in. If I get time today, I'd like to copy the ROM to my NeoSD and try it on my real AES console!

UPDATE: Scramble running on my Neo Geo AES!

Washed out colour due to the phone camera/flash